Fire detection devices are such as smoke detectors and/or gas detectors are generally employed in structures or machines to monitor the environmental conditions within the living area or occupied compartments of a machine. These devices typically provide an audible or visual warning upon detection of a change in environment conditions that are generally accepted as a precursor to a fire event.
Typically, smoke detectors include a smoke sensing chamber, exposed to the area of interest. The smoke detector's smoke sensing chamber is coupled to an ASIC or a microprocessor circuit. The smoke sensor samples the qualities of the exposed atmosphere and when a change in the atmosphere of the exposed chamber is detected by the microprocessor, an alarm is sounded.
There are two types of smoke sensors that are in common use. Optical or photoelectric type smoke sensors and ionization type smoke sensors. Photoelectric-based detectors are based on sensing light intensity that is scattered from smoke particles. Light from a source (LED) is scattered and sensed by a photosensor. When the sensor detects a certain level of light intensity, an alarm is triggered.
Ionization-type smoke detectors are typically based on a radioactive material that ionizes some of the molecules in the surrounding gas environment. The current of the ions is measured. If smoke is present, then smoke particles neutralize the ions and the ion current is decreased, triggering an alarm.
The ionization smoke detectors that are currently available in the market are very sensitive to fast flaming fires. This type of fire produces considerable energy and ionized particles and is easily detected by the sensor.
Although the ionization technology is very inexpensive compared with other technologies and had been installed in millions of homes, there is discussion regarding phasing out of this product category. It has been suggested by some members of the National Fire Protection Agency (NFPA) that ionization smoke sensors do not readily detect smoldering fires.
Smoldering fires most commonly, result from cigarette ignition of materials found in homes such as sofas and beds. A smoldering fire typically produces cold smoke particles of which only a small portion is ionized. Because ionization technology focuses on detection of ionized particles, smoldering fire detection may be inconsistent.
A variety of optical gas sensors for detecting the presence of hazardous gases, especially carbon monoxide (“CO”), are also known.
Typically, optical gas sensors include a self-regenerating, chemical sensor reagent impregnated into or coated onto a semi-transparent substrate. The substrate is typically a porous monolithic material, such as silicon dioxide, aluminum oxide, aluminosilicates, etc. Upon exposure to a predetermined target gas, the optical characteristics of the sensor change, either darkening or lightening depending on the chemistry of the sensor.
Smoke and gas sensors can be affected by temperature, humidity, and dust particles. One or a combination of these ambient factors can cause a smoke or gas detector to false alarm.